1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst.
2. Description of the Related Art
Emission controls on automobiles, etc. have been tightened in recent years. Therefore, in order to address this issue, various exhaust gas-purifying catalysts for purifying hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), etc. contained in an exhaust gas have been developed.
Most exhaust gas-purifying catalysts contain precious metals as catalytic metals. The precious metals play a role in promoting oxidation reactions of HC and CO and reduction reactions of NOx.
The precious metals, however have a problem that they are susceptible to poisoning by HC in the exhaust gas. When the precious metals are poisoned by HC, the reduction reaction of NOx on these precious metals becomes less prone to occur. Therefore, a catalyst containing precious metals sometimes shows an insufficient NOx purification efficiency specifically under a rich atmosphere in which the exhaust gas contains HC at a high concentration.
In order to suppress this, proposed is a technique in which a salt of an alkaline-earth metal element is added to a catalyst containing these precious metals (see, for example, Patent document 1). When such a composition is employed, the alkaline-earth metal element acts to suppress the poisoning of the precious metals by HC, and thus exhaust gas purification performances such as an NOx purification efficiency can be improved to some extent.
The exhaust gas-purifying catalysts, however, have room for further improvement in the exhaust gas purification performances.